1. Field of the Invention
This invention relates to radar test systems, and more particularly to test systems capable of simulating target movement relative to the radar set.
2. Description of the Related Art
Modern radar systems employ complex waveforms and need to have their performance completely tested (from end to end) under controlled conditions. Test systems used to simulate a moving target are generally designed to return a copy or simulated copy of a transmitted radar pulse, after delaying or simulating the delay of the pulse by a period of time corresponding to the round-trip transit time to the target, with the addition of an expected Doppler frequency shift. The delay parameters are updated at regular intervals, corresponding to the target velocity and the desired range resolution, to simulate target movement.
The general approach to testing using delayed transmitted radar pulses has been to put a large target (such as a corner reflector or repeater) out at a distance from the radar so that a long delay is achieved because of the round trip time to this simulated target. This technique suffers from the fact that the radar signal is perturbed by the outside environment that is not under the control of the tester, and has several undesirable features. Excessive losses at radio frequency (RF) rule out the feasibility of using coaxial cables to provide long delay times. Surface acoustic wave delay lines are also lossy, with a limited RF bandwidth and delay time capability. Simulated target generators for radar testing can employ digital RF memory (DRFM) to simulate a moving target. Such a system employs sampling and analog-to-digital conversion techniques to store sampled waveform data of the transmitted radar pulses. However, DRFM introduces undesired frequency spurs (spectral impurities), has a reduced dynamic range and bandwidth because of the need for analog-to-digital conversion, and requires complex circuitry that has a relatively high power consumption. Also an RF target generator system can generate a simulated delayed radar pulse by using reference signals from the radar to generate the pulse in the same way the radar generates the signal that goes to the radar transmitter.
A recirculating fiber optic delay line/memory has also been proposed in P. R. Herczfeld, I. Koffman, A. S. Daryoush, R. Saedi, B. Even-Or and R. Markowitz, "A Fiberoptic Recirculating Delay Line", Proc. SPIE, Vol. 996, pp. 116-123 (1988). In this type of system the RF signal circulates repeatedly around a fiber optic loop, and is tapped off after the desired delay period. Although it employs a regenerative process to overcome losses, the noise buildup associated with the regeneration limits the number of recirculations and the technique needs perfecting.
A fixed fiber optic delay line is described in U.S. Pat. No. 4,903,029 to Newberg et al., and assigned to Hughes Aircraft Company, the assignee of the present invention. While this is a low loss delay mechanism, it incorporates a fixed delay that cannot be adjusted, and therefore can be used to simulate only a fixed target distance without any target movement.